Diuretics are a group of drugs used to treat a variety of medical conditions, including congestive heart failure, hypertension, certain types of liver and kidney diseases and increased intra-ocular pressure. Diuretics act on the transport of sodium (Na+) by the nephrons of the kidney so as to increase the renal excretion of Na+ (and associated ions) and water out of the body and thereby to decrease the extracellular fluid (ECF) volume. Normally, Na+ enters the ECF via the diet, and is excreted in the urine in amounts identical to the intake. In normal adults over 99% of the sodium entering the nephrons of the two kidneys (via glomerular filtration) is transported via an energy dependent process out of the tubular fluid and back into the ECF. When this balance between intake and excretion is upset (with excretion falling below intake) salt retention will occur. A primary mechanism of treating this abnormality involves the administration of one or more agents that reduce Na+ and water reabsorption by the kidneys and thereby increase their excretion in the urine. These agents, collectively, are known as diuretics. Optimally, a powerful diuretic should be natriuretic (inhibit resorption of sodium ions) but not kaliuretic (inhibit resorption of potassium ions) since potassium loss is an undesirable side effect. The principle drugs which are included in the “diuretic” category act by inhibiting the transport of Na+ (and water) out of the tubular fluid by acting on a specific “carrier” in the tubular epithelial cells at a specific site of the nephron. The latter varies with the diuretic employed.
Several major classes of diuretics exist including loop diuretics, thiazide-type diuretics and potassium-sparing diuretics. Loop diuretics, also known as high-ceiling diuretics, act on the thick ascending loop of Henle within the kidney. Examples include furosemide, bumetanide and toresemide. Loop diuretics have a peak diuretic effect far greater than other classes of diuretics. This class acts to inhibit electrolyte reabsorption resulting in the excretion of not only sodium, but also potassium, calcium and magnesium. Loop diuretics are considered “potassium wasting.” For example, furosemide is commonly used to treat heart failure, pulmonary edema, hypertension and poisoning. Unfortunately, if dietary potassium is not sufficient, hypokalemia may result and this may induce cardiac arrythmias (Goodman and Gilman's The Pharmacological Basis for Therapeutics, 10th Ed.; Hardman, Limbird & Gilman, Eds. MacGraw-Hill, p. 772, 2001).
Thiazide-type diuretics act in the distal tubule and connecting segment of the kidneys. Examples include chlorothiazide, chlorthalidone, hydrochlorothiazide, indapamide and metolazone. Although thiazides cause less distortion of the electrolyte composition of the extra-cellular fluid than other classes of diuretics, there is also lower intensity of diuresis produced by these drugs. This class contains many sulfonamide chemical entities and thus may cause an allergic reaction in those with sulfa allergies. Although thiazides do not cause calcium excretion, potassium excretion increases with acute administration. Thiazides may also induce hyperglycemia and aggravate pre-existing diabetes mellitus. Thiazide diuretics may also cause increased serum cholesterol, low-density lipoprotein (LDL) and triglyceride concentration. Thiazides are also considered “potassium wasting” diuretics.
Potassium-sparing diuretics may act through either of several mechanisms. Some are steroidal in structure and act in aldosterone-sensitive cells in the cortical connecting tubule in the kidney. Members in this drug class are competitive antagonists of endogenous mineralocorticoid steroids such as aldosterone, which acts to enhance sodium absorption and potassium excretion. The aldosterone receptor is a soluble, cytoplasmic protein found in several tissues including salivary glands, colon and segments of nephrons in the kidney. Spironolactone, a representative member of this drug class, binds to the aldosterone receptor and prevents the receptor from assuming an active conformation. Spironolactone also increases calcium excretion. Common side-effects includes nausea, stomach cramps and diarrhea. Other side effects involve endocrine imbalances, gynecomastia (abnormal enlargement of one or both breasts in men), altered libido, impotence or hirsutism (excessive body hair). Triamterene and amiloride are non-steroidal potassium-sparing diuretics that inhibit electrogenic entry of sodium in the late segments of the kidney nephron. Triamterene and amiloride cause an increase in sodium and chloride excretion, but have little effect on potassium excretion. Side effects of Triamterene include hyperkalemia (increased serum potassium concentration), nausea, vomiting, leg cramps and dizziness. Amiloride side effects also include hyperkalemia, nausea, vomiting, diarrhea and headache.
Other classes of diuretics include osmotic diuretics and carbonic anhydrase inhibitors. Osmotic diuretics, such as mannitol, are poorly reabsorbed by the renal tubules. This drug class effects poor net reabsorption of sodium salts. In addition, mannitol is poorly absorbed by the gastrointestinal tract, and thus must be administered intravenously. Other osmotic diuretics include glycerol, urea and isosorbide.
Carbonic anhydrase inhibitors, such as acetazolamide, cause a modest decrease of sodium reabsorption and may also cause loss of potassium and metabolic acidosis due to its mechanism of action.
Diuretics are used to treat high blood pressure (hypertension), either alone, or in combination with other drugs. High blood pressure adds to the workload of the heart and arteries. If the condition continues for a prolonged period of time, heart and artery function may be impaired. This can damage the blood vessels of the brain, heart and kidneys, resulting in stroke, heart or kidney failure. High blood pressure may also increase the risk of heart attack. These risks can be reduced if blood pressure is properly controlled. The National Heart, Lung, and Blood Institute's (NHLBI) high blood pressure guidelines (JAMA, May 21, 2003; www.nhlbi.nih.gov) emphasize a need to develop new diuretic medications without the side affects of the aforementioned diuretic pharmacopeia.